That message can have many possible causes:
- The charging adapter truly is not compatible with iOS devices. Apple only added a test for compatibility in iOS 8, after a number of questionable devices destroyed iPhones and iPads.
- The charging adapter may have failed (as I mentioned)
- The cable has a fault - try a different cable
- The cable is not an Apple cable or one certified by Apple as "Made for iPhone"
- The Lightning plug has dirt or corrosion on it.
- There is dirt in the Lightning connector on the phone (less likely if the problem only occurs in the car)
So the first step is to get a new car adapter. They're inexpensive. Belkin and InCase are good, reliable brands, and Apple sells them in the online Apple store. You can also try different cables. For example, try switching the cable you use in the car with the one you use in the house.